Gallium nitride based compound semiconductor (hereinafter to be also referred to as “GaN-based semiconductor”) is compound semiconductor made of a III group nitride represented by the chemical formula:AlaInbGa1-a-bN(0≦a≦1, 0≦b≦1, 0≦a+b≦1),which is exemplified by those having any composition such as GaN, InGaN, AlGaN; AlInGaN, AlN, InN and the like. A compound semiconductor of the above-mentioned chemical formula wherein a part of the III group element is substituted by B (boron), Tl (thallium) and the like and one wherein a part of N (nitrogen) is substituted by P (phosphorus), As (arsenic), Sb (antimony), Bi (bismuth) and the like are encompassed in the GaN-based semiconductor.
In recent years, GaN-based semiconductor light emitting elements such as light emitting diode (LED), laser diode (LD) and the like, which emit light having a wavelength of from green to ultraviolet, have been practiced and are drawing attention. This light emitting element has, as a basic structure, a pn junction diode structure formed by joining an n-type GaN-based semiconductor and a p-type GaN-based semiconductor. Simply put, according to the emission mechanism of the light emitting element, when electron injected into n-type GaN-based semiconductor and positive hole injected into p-type GaN-based semiconductor are recombined together to lose energy in the pn junction or the vicinity thereof, the light corresponding to the energy is emitted. In such an element, an electrode that makes an ohmic contact with an n-type GaN-based semiconductor (hereinafter to be also referred to as “n-type ohmic electrode”) is used to efficiently inject electron into an n-type GaN-based semiconductor. In LED, a constitution wherein an n-type ohmic electrode also serves as a contact electrode is generally employed. A contact electrode is an electrode to which a bonding wire, a solder and the like used for electric connection of an element and an external electrode for the element are bonded. A contact electrode is required to show a good bondability with a bonding wire (e.g., Au wire) or a solder (e.g., Au—Sn eutectic). When the bondability is poor, a defect tends to occur in a chip mounting process.
Conventionally, a single layer film of Al (aluminum) or a multi-layer film wherein an Al layer is laminated on a Ti (titanium) layer has been used as an n-type ohmic electrode (JP-A-7-45867, U.S. Pat. No. 5,563,422). However, since these electrodes are mainly made of an Al layer, they show low heat resistance and, for example, problematically easily deformed when a heat treatment is applied. This is caused by the facts that Al has a low melting point, thermal stress is easily developed inside the electrode because the thermal expansion coefficient of Al is extremely large as compared to that of GaN-based semiconductor, and the like. Moreover, when these electrodes are used as contact electrodes, an oxidized film is formed on the surface of Al, which degrades Au wire bondability and wettability by Au—Sn eutectic solder. Therefore, the yield tends to be low in a chip mounting process. To solve this problem, an electrode wherein an Au layer is laminated on an Al layer at a layer made of a metal having a comparatively high melting point has been proposed (JP-A-7-221103, U.S. Pat. No. 5,563,422). However, this electrode also requires a heat treatment at a temperature of about 400° C. to decrease the contact resistance, since it is in contact with an n-type GaN-based semiconductor at an Al layer. The heat treatment roughens the surface of the electrode and problematically degrades the bondability of the electrode with bonding wire or solder. This electrode is associated with the problem in that production of electrodes with the same property with good reproducibility is difficult, since the contact resistance with n-type GaN-based semiconductor after a heat treatment is affected by the diffusion state of Al and Au due to the thermal stress.
As an n-type ohmic electrode free of Al, JP-A-11-8410 discloses an n-type ohmic electrode obtained by laminating TiW alloy layer, Ge (germanium) layer and Rh (rhodium) layer and heat-treating the laminate. The principle of the good ohmic contact formed by the electrode with the n-type GaN-based semiconductor is unclear. However; since a good ohmic contact is formed irrespective of the order of lamination of these three metal layers, it is postulated that a product resulting from the chemical reaction involving all three metal layers plays some role. Therefrom it is expected that the property of the obtained electrode will not be stabilized unless the conditions under which the three layers are laminated and the conditions of the subsequent heat treatment in the formation of the electrode are strictly controlled. Therefore, a semiconductor element using this electrode is considered to be unsuitable for a large-scale production.